A new study solidifies the connection between breast-feeding and intelligence, but it’s not the only way that mom can influence youngsters’ IQ, even before birth.

While previous studies have drawn a link between breastfeeding and cognition, and even extended an association to higher social status, it’s never been absolutely clear whether the connection was due to the breast milk or to the bond that the practice builds between mom and child, which can itself enhance brain development. Or, for that matter, whether it was mom’s (and dad’s) own education and social status that filtered through in their parenting that contributed to their children’s IQ.

But researchers from Boston Children’s Hospital, reporting in JAMA Pediatrics, say they have conducted the most comprehensive study to date to flesh out how much breast feeding influences brain development in youngsters. They teased away as many factors known to contribute to intelligence as possible, such as mother’s IQ, parental income and education, and whether the toddlers spent time in child care outside of the home. They also collected detailed information from 1,312 babies and mothers on how long the moms breast fed their babies, whether the infants were exclusively breastfed, and how much fish the mothers consumed, since omega-3 fatty acids found in some fish are also present in breast milk and known to influence brain development.

The longer the mothers breastfed, the more likely their children were to score higher on vocabulary tests at age 3 and on intelligence tests at age 7. Breastfeeding during a child’s first year of life could boost their IQ by about four points when they enter school.

Was it the breast milk? Some studies are starting to focus on the nutrients, such as the fatty acids docosahexaenoicacid (DHA) and arachidonic acid (ARA), present in breast milk, but there’s no solid evidence that these are responsible for enhancing neural connections or promoting development of cognitive regions of the brain.

Even without that evidence, the latest findings will likely fuel efforts to promote exclusive breastfeeding, which have been growing in recent decades. The American Academy of Pediatrics and the World Health Organization support advise new mothers to breastfeed exclusively for the first six months of their babies’ lives, and continue for at least one year. But in the U.S., many mothers who start the practice don’t maintain it for the full fix months. “The problem currently is not so much that most women do not initiate breast-feeding, it is that they do not sustain it. In the United States about 70% of women overall initiate breast-feeding… However, by six months, only 35% … are still breast-feeding,” writes Dr. Dimitri Christakis of the Seattle Children’s Hospital Research Institute in an editorial accompanying the study.

But it’s not just breastfeeding that can have an impact on a child’s cognitive development. It turns out that expectant moms can have a significant influence on her baby’s intelligence thanks to the air she breathes and the foods she eats. That’s in addition to the influence that she and other family members continue to have on brain development during a child’s life:

Genes: Researchers have used genome-wide analyses and brain imaging to identify gene variants that parents pass along to their children that may be involved in brain size and IQ. In April 2012, scientists from the Enhancing Neuro Imaging Genetics through Meta-Analysis (ENIGMA) Consortium identified two genes that impact both brain size and IQ–but also raise the risk for developing brain disorders like Alzheimer’s disease.

Pollution: A mother’s exposure to pollutants such lead, mercury and those found in car exhaust and pesticides during pregnancy has been connected to everything from ADHD to autism to lower IQ in children.

BPA: Studies in animals found that pregnant mice exposed to bisphenol A gave birth to offspring with altered nerve development, which could translate into neurodevelopmental disorders.

Birth Order: A 2007 study reported that the oldest child in a family has about a three point higher IQ on average compared to the next closest sibling. Why? Parents may spend more time engaging their first child, either by reading or playing games that stimulate their cognitive development.

Junk Food: University of Bristol researchers found that toddlers who ate significant amounts of processed food at age three were more likely to have lower IQs by age eight. For every unit increase in processed food in their diets, the kids in the study lost 1.67 points in IQ. But for each unit increase in healthy foods such as fruits, vegetables and fish, kids’ IQs increased by 1.2 points.

Spanking: Studies have found a correlation between children who are spanked and lower IQ scores. While the trauma of corporal punishment can lead to stress and fear that interferes with learning, it’s also possible that children with learning problems who have lower IQs tend to misbehave and elicit more punishment.